Nature and significance of small volume fall deposits at composite volcanoes: Insights from the October 14, 1974 Fuego eruption, Guatemala

The first of four successive pulses of the 1974 explosive eruption of Fuego volcano, Guatemala, produced a small volume (similar to 0.02 km(3) DRE) basaltic sub-plinian tephra fall and flow deposit. Samples collected within 48 h after deposition over much of the dispersal area (7-80 km from the volcano) have been size analyzed down to 8 phi (4 mu m). Tephra along the dispersal axis were all well-sorted (sigma(phi)=0.25-1.00), and sorting increased whereas thickness and median grain size decreased systematically downwind. Skewness varied from slightly positive near the vent to slightly negative in distal regions and is consistent with decoupling between coarse ejecta falling off the rising eruption column and fine ash falling off the windblown volcanic cloud advecting at the final level of rise. Less dense, vesicular coarse particles form a log normal sub-population when separated from the smaller (Md-phi < 3 phi or < 0.125 mm), denser shard and crystal sub-population. A unimodal, relatively coarse (Md-phi=0.58 phi or 0.7 mm sigma(phi)=1.2) initial grain size population is estimated for the whole (fall and flow) deposit. Only a small part of the fine-grained, thin 1974 Fuego tephra deposit has survived erosion to the present day. The initial October 14 pulse, with an estimated column height of 15 km above sea level, was a primary cause of a detectable perturbation in the northern hemisphere stratospheric aerosol layer in late 1974 to early 1975. Such small, sulfur-rich, explosive eruptions may substantially contribute to the overall stratospheric sulfur budget, yet leave only transient deposits, which have little chance of survival even in the recent geologic record. The fraction of finest particles (Md-phi=4-8 phi or 4-63 mu m) in the Fuego tephra makes up a separate but minor size mode in the size distribution of samples around the margin of the deposit. A previously undocumented bimodal-unimodal-bimodal change in grain size distribution across the dispersal axis at 20 km downwind from the vent is best accounted for as the result of fallout dispersal of ash from a higher subplinian column and a lower "co-pf" cloud resulting from pyroclastic flows. In addition, there is a degree of asymmetry in the documented grain-size fallout pattern which is attributed to vertically veering wind direction and changing windspeeds, especially across the tropopause. The distribution of fine particles (< 8 mu m diameter) in the tephra deposit is asymmetrical, mainly along the N edge, with a small enrichment along the S edge. This pattern has hazard significance.